1. Field of the Invention
The present invention relates to an image pickup lens for forming an optical image of a subject on an image sensor, such as a CCD (charge coupled device), a CMOS (complementary metal oxide semiconductor), or the like, and an image pickup apparatus, such as a digital still camera or the like, having the image pickup lens mounted thereon to perform imaging. The invention also relates to a portable terminal device, such as a camera-equipped cell phone, a personal digital assistance (PDA), or the like.
2. Description of the Related Art
Recently, along with the spread of personal computers to homes and the like, digital still cameras capable of inputting image information obtained by imaging a landscape, a person, or the like to a personal computer have been spreading rapidly. In addition, more and more cell phones have built-in camera modules for image input. Such devices with imaging capabilities employ image sensors such as CCDs, CMOSs, and the like. In recent years, these types of image sensors have been downsized greatly and, consequently, imaging devices as a whole and image pickup lenses to be mounted on the devices have also been required to have more compact sizes. At the same time, the pixel count of image sensors has been increasing, thereby causing a growing demand for improvement of image pickup lenses in resolution and performance.
Image pickup lenses having a two lens configuration are disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-284003, Japanese Unexamined Patent Publication No. 2006-208826, U.S. Pat. No. 7,535,660, Japanese Unexamined Patent Publication No. 2008-152004, and Japanese Unexamined Patent Publication No. 2009-075437.
As described above, downsizing and pixel count increase have been in progress for recent image sensors. For image pickup lenses of portable camera modules, in particular, cost reduction and compactness have been the major demands, but as the pixel count of image sensors even for portable camera modules tends to be increased, a demand for performance improvement is also growing. Consequently, development of wide variety of lenses comprehensively taking into account the cost, performance, and compactness is anticipated.
For example, an image pickup lens with a relatively wide viewing angle, a bright F-number of about 2.8, and a reduced total length is required. Consequently, it is demanded that the total lens length (distance from the most object side surface to the image plane) with respect to the focal length is reduced. In the mean time, if a lens comes too close to a sensor surface due to the recent downsizing of image sensors, dirt adhered to the surface of the lens affects largely on the light intensity variation, it is, therefore, necessary to secure a relatively long back focus as a countermeasure. Consequently, it has been anticipated that an image pickup lens formed of only two lenses, yet having a relatively wide viewing angle of, for example, 50 to 80 degrees in terms of horizontal angle of view, a bright F-number of about 2.8, a reduced total length, and a back focus of certain extent is developed. In order to deal with high pixel count image sensors, image pickup lenses are required to have telecentricity, that is, the incident angle of the chief ray becomes substantially parallel to the optical axis (incident angle on the image plane becomes close to zero with respect to normal line). Consequently, it is also required that the exit pupil position is set at a position remote from the imaging plane while the total length of the image pickup lens is reduced. Further, it is naturally demanded that various aberrations, including spherical aberration, distortion, chromatic aberration, and the like, are sufficiently corrected while the aforementioned requirements are satisfied.
Consequently, development of two lens image pickup lenses having higher performance than that of conventional two lens image pickup lenses has been sought. For example, the image pickup lenses described in Japanese Unexamined Patent Publication No. 2005-284003 and in Japanese Unexamined Patent Publication No. 2006-208826 have a problem that the total length is too long. The image pickup lens described in Japanese Unexamined Patent Publication No. 2008-040413 has a problem that the total length is too short having difficulty in ensuring the telecentricity. The image pickup lens described in Japanese Unexamined Patent Publication No. 2008-152004 has a problem that the back focus is too short and may cause light intensity variation. For the image pickup lens described in Japanese Unexamined Patent Publication No. 2009-075437, disclosure of specific lens data and aberration performance is insufficient.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an image pickup lens reduced in total length with a small number of two lenses while having advantages in ensuring wide angle of view, brightness, and back focus with high image forming performance. It is a further object of the present invention to provide an image pickup apparatus and a portable terminal device, each having the image pickup lens and being capable of obtaining a high resolution image pickup signal.